1. Field of the Invention
The present invention relates to a planar inductor.
2. Description of the Related Art
Conventionally known are planar inductors in which two spiral conductor coils 1a and 1b are sandwiched between ferromagnetic ribbons 2a and 2b with insulating layers 3a, 3b and 3c alternately interposed between them, as shown in FIG. 1. FIG. 1A is a plane view of one such prior art planar inductor, and FIG. 1B is a sectional view of the inductor as taken along line A--A of FIG. 1A. Full and broken lines in the plane view of FIG. 1A, which are indicative of conductor coils 1a and 1b, respectively, correspond to the respective center lines of coils 1a and 1b shown in the sectional view of FIG. 1B. Insulating layers 3a, 3b and 3c are formed of a dielectric or the like. Coils 1a and 1b are connected electrically to each other via through hole 4, and form an inductor between terminals 5a and 5b at their respective end portions.
If a current is applied to spiral conductor coils 1a and 1b of the planar inductor, however, magnetic fluxes 6a and 6b flow in opposite directions from the center or through-hole 4, as shown in FIG. 2. As a result, gap portions 7a and 7b, where magnetic flux density is very low, exist at two positions near the central and outer peripheral portions of each conductor coil. Accordingly, the inductance is inevitably reduced. In this case, an intensive magnetic field is generated at central gap portion 73 by conductor coils 1a and 1b, while there is hardly any magnetic field at peripheral gap portion 7b. Thus, the reduction of the inductance is much greater at the peripheral portion than at the central portion.
Spiral conductor coils 1a and 1b, insulating layers 3a, 3b and 3c, and ferromagnetic ribbons 2a and 2b, which constitute the planar inductor, must be bonded together. If insulating layers 3a, 3b and 3c are formed from an organic polymer, for example, the individual layers may be bonded by being pressurized at a temperature not lower than the softening point of the material, or otherwise, the contact portions between the elements may be bonded by means of a suitable bonding agent.
If magnetostriction of ferromagnetic ribbons 2a and 2b is substantial, however, compressive stress or other stress acts on the surfaces of the ribbons while adjacent insulating layers 3a, 3b and 3c are being bonded. Interactions of the stress and the magnetostriction deteriorates the magnetic characteristics, thereby lowering the effective permeability. If ferromagnetic ribbons 2a and 2b are subject to strain during use of the completed planar inductor, the effective permeability also changes, so that the inductance may possibly vary. The higher the permeability, the more noticeable these phenomena will be.
In a magnetic circuit cf this planar inductor, if ferromagnetic ribbons 2a and 2b are thicker, then the magnetic resistance is generally reduced in proportion, thus increasing the inductance. However, this is inconsistent with the object to minimize the general thickness of the plane inductor.
Meanwhile, the planar inductor may be applied to an output-side choke coil of a DC-DC converter or the like. In this case, a high-frequency current superposed with a DC current flows through the planar inductor. Therefore, the inductor requires a good DC superposition characteristic.
The conventional planar inductors have not, however, a very good DC superposition characteristic. If this characteristic of the inductor is poor, the inductance lowers, so that the control becomes difficult. Accordingly, the efficiency of the DC-DC converter lowers. Thus, it is not appropriate to apply the plane inductor directly to the DC-DC converter and the like.